You simply forgot to return $txt. A sub returns the result of the last statement by default, in your case this is the substitution. Try this instead:

#!/usr/local/bin/perl -w
use strict;
my $txt="This is a line\nThis is a line as well\n\nThis is the last li
+ne\n";
print ubb_txt($txt);
sub ubb_txt {
  my $txt = shift;
  return "" unless ( defined $txt);
  my ($nl)=$txt =~ m{(\cM?\cJ)};
  $txt =~ s{$nl$nl}{</p><p>}gs if (defined $nl);
  $txt =~ s{$nl}{<br>}gs if (defined $nl);
  return $txt;
}
__END__
This is a line<br>This is a line as well</p><p>This is the last line<b
+r>
[download]

Your sub was returning the number of substitutions made (i.e. the result of the last substitution).

CU
Robartes-