grep is normally used to return a list, like this:

my @new_list = grep { /regular expression/ } @list;
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now, @new_list contains every value in @list that matches /regular expression/.

Your code:

($i)= grep { m/^#/ } $_;
push @new, $i;
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Will set $i to undefined if $_ doesn't start with a # character, and then push it onto @new.

In other words you'll have undefined entries in @new where you probably wanted to skip those lines.

You might want to do some reading in the excellent perl documentation

updated: fixed link