use strict;

my $text = <<TEXT;
<p>Before new line.
After new line.</p>
TEXT

print "Before: |$text|\n";

$text=~s#<p>(.+?)<\/p>#"<p>".&enter($1)."<\/p>"#gsie;

print "After: |$text|\n";


sub enter
{
my $new;

($new)=$1;
$new=~s/\n//gsi;
return $new;
}