$B .= "$1\n" unless $1 =~ /def/;

##</code><code>##

$B .= "$1\n" if $match =~ /def/;

##</code><code>##

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