#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;

my $B = "";
my $A = "abc
def
ghi
";

while ( $A =~ m{(.+)}g ) {
   $B .= "$1\n" unless $1 =~ /def/;
}
print "\$B = \n$B\n";


$B = "";
while ( $A =~ m{(.+)}g ) {
   $B .= "$1\n" while $1 =~ /def/;
}
print "\$B = \n$B\n";