#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;

my $A = "abcdef
fhijkl
mnopqr
redefine
";

( my $B = $A ) =~ s[(^.+\n)][ my $x = $1; $x !~ /def/ ? '' : $x ]gme;

print "$B";

__END__
C:\test>1145825.pl
abcdef
redefine