$bar =~ s#(\d{2})#"<td>".foo($1)."</td>"#mesg;

##</code><code>##

#!perl -w
 sub foo { "<$_[0]>" };
 my $bar = '2299-33-ff';
 $bar =~ s#(\d{2})#"<td>".foo($1)."</td>"#mesg;
 print $bar;
__END__
<td><22></td><td><99></td>-<td><33></td>-ff