I just took your code as a starting point and modified it a bit. It would appear that just checking whether a decimal exists or not is sufficient. Take out that if logic if you want all data to be formatted like $x.yy.

#!/usr/bin/perl -w
use strict;

foreach my $in  qw(-10000000 1000.000 10.000 9.999 10 10.1 0 12345 5.)
{
   print "in = $in \tformatted = ",f_currency($in),"\n";
}

sub f_currency 
{
   my $num = shift;

   my $rounded = $num;
   if ($num =~ m|\.|)
   {
      $rounded = sprintf("%.2f", $num); #rounds to 2 dec places
   }
   
   #add commas, all the "work" happens in the while condition
   while ($rounded =~ s/^(-?\d+)(\d\d\d)/$1,$2/){};

   return ('$'.$rounded);
}

__END__
prints:
in = -10000000  formatted = $-10,000,000
in = 1000.000   formatted = $1,000.00
in = 10.000     formatted = $10.00
in = 9.999  formatted = $10.00
in = 10     formatted = $10
in = 10.1   formatted = $10.10
in = 0  formatted = $0
in = 12345  formatted = $12,345
in = 5.     formatted = $5.00
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