One line script to edit after the Pattern

I got an example here but wouldn't write the output/print to a file

perl -lne 'print $_;print "This is Middle" if(/Beginning/);' file

I tried putting > file but it happens to overwrite it all empty.

Solution

You can't read from a file and write to it at the same time. The usual way to handle this kind of thing is write to a temporary file and replace the original with it after the action:

perl -lne 'print $_;print "This is Middle" if(/Beginning/);' file > file.tmp; \
mv -f file.tmp file