How does `print readline` differ from `$line = readline; print $line`?

Question

I wrote a script that prints the current and subsequent line after a match, however, I've noticed a difference between two ways of using the readline return value.

This works as intended:

$ echo $'H1\na\nb\nH2\na\nb' | perl -ne 'if (/^H/) { print; print $line = readline }'
H1
a
H2
a

However, this doesn't:

$ echo $'H1\na\nb\nH2\na\nb' | perl -ne 'if (/^H/) { print; print readline }'
H1
a
b
H2
a
b

instead, it prints all the lines. Why?

Answer 1

print takes a list argument, and puts readline() in list context. Hence it exhausts all of the lines. When you use scalar assignment with $line = it is in scalar context, and will only read 1 line.

Read more about context here.