Differences between empty and blank lines in regexps

There are already several good discussions of regular expressions and empty lines on SO. I'll remove this question if it is a duplicate.

Can anyone explain why this script outputs 5 3 4 5 4 3 instead of 4 3 4 4 4 3? When I run it in the debugger $blank and $classyblank stay at "4" (which I assume is the correct value) until the just before the print statement.

my ( $blank, $nonblank, $non_nonblank, 
     $classyblank,  $classyspace, $blanketyblank ) = 0 ;

while (<DATA>) {

  $blank++ if /\p{IsBlank}/         ; # POSIXly blank - 4?
  $nonblank++ if /^\P{IsBlank}$/    ; # POSIXly non-blank - 3
  $non_nonblank++ if not /\S/       ; # perlishly not non-blank - 4
  $classyblank++ if /[[:blank:]]/   ; # older(?) charclass blankness - 4?
  $classyspace++ if /^[[:space:]]$/ ; # older(?) charclass whitespace - 4
  $blanketyblank++ if /^$/          ; # perlishly *really empty*  - 3

}

print join " ", $blank, $nonblank, $non_nonblank,
            $classyblank, $classyspace, $blanketyblank , "\n" ;

__DATA__

line above only has a linefeed this one is not blank because: words

this line is followed by a line with white space (you may need to add it)

then another blank line following this one

THE END :-\

Is it something to do with the __DATA__ section or am I misunderstanding POSIX regular expressions?

ps:

As noted in comment on a timely post elsewhere, "really empty" (/^$/) can miss non-emptiness:

perl -E 'my $string = "\n" . "foo\n\n" ; say "empty" if $string =~ /^$/ ;'
perl -E 'my $string = "\n" . "bar\n\n" ; say "empty" if $string =~ /\A\z/ ;'
perl -E 'my $string = "\n" . "baz\n\n" ; say "empty" if $string =~ /\S/ ;'

Solution

/\p{IsBlank}/ doesn't check for a empty string. \p matches a character that has the specified Unicode property.

$ unichars '\p{IsBlank}' | cat
 ---- U+0009 CHARACTER TABULATION
 ---- U+0020 SPACE
 ---- U+00A0 NO-BREAK SPACE
 ---- U+1680 OGHAM SPACE MARK
 ---- U+2000 EN QUAD
 ---- U+2001 EM QUAD
 ---- U+2002 EN SPACE
 ---- U+2003 EM SPACE
 ---- U+2004 THREE-PER-EM SPACE
 ---- U+2005 FOUR-PER-EM SPACE
 ---- U+2006 SIX-PER-EM SPACE
 ---- U+2007 FIGURE SPACE
 ---- U+2008 PUNCTUATION SPACE
 ---- U+2009 THIN SPACE
 ---- U+200A HAIR SPACE
 ---- U+202F NARROW NO-BREAK SPACE
 ---- U+205F MEDIUM MATHEMATICAL SPACE
 ---- U+3000 IDEOGRAPHIC SPACE

It matches " \n" since SPACE has the IsBlank property.

/[[:blank:]]/ doesn't check for a empty string. [...] matches a character that is a member of the specified class.

$ unichars '[[:blank:]]' | cat
 ---- U+0009 CHARACTER TABULATION
 ---- U+0020 SPACE
 ---- U+00A0 NO-BREAK SPACE
 ---- U+1680 OGHAM SPACE MARK
 ---- U+2000 EN QUAD
 ---- U+2001 EM QUAD
 ---- U+2002 EN SPACE
 ---- U+2003 EM SPACE
 ---- U+2004 THREE-PER-EM SPACE
 ---- U+2005 FOUR-PER-EM SPACE
 ---- U+2006 SIX-PER-EM SPACE
 ---- U+2007 FIGURE SPACE
 ---- U+2008 PUNCTUATION SPACE
 ---- U+2009 THIN SPACE
 ---- U+200A HAIR SPACE
 ---- U+202F NARROW NO-BREAK SPACE
 ---- U+205F MEDIUM MATHEMATICAL SPACE
 ---- U+3000 IDEOGRAPHIC SPACE

It matches " \n" since SPACE is a member of the [:blank:] POSIX character class and thus a member of the [[:blank:]] character class.