I think the right hand side in an s///e block must be valid Perl. <td>foo($1)</td> is not valid Perl.

Quoting the HTML tags should work:

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