use strict;
use Template ();
use Scalar::Util qw( looks_like_number );

sub fmt_currency {
  my( $txt ) = @_;
  return $txt unless looks_like_number( $txt );
  
  if( $txt < 0 ) {
    $txt = '<span class="neg">(' . abs( $txt ) . ')</span>'
  } else {
    ## maybe wrap $txt with a pos span . . .
  }
  
  return $txt;
}

my $t = Template->new(
  FILTERS => { fmt_currency => \&fmt_currency },
  ## ...
);
my $data = { qw/foo 1.23 bar -3.45 baz quux/ };

my %tmpl_src = <<EOT;
[% foo | fmt_currency %] [% bar | fmt_currency %] [% baz | fmt_currency %]
EOT

$t->process( \$tmpl_src, $data, \*STDOUT );

exit 0;

__END__
1.23 <span class="neg">(3.45)</span> quux