As someone said, it's not easy in general, since you can embed various perlisms into Perl regexes that POSIX regexes have no way of knowing about (lacking a perl interpreter).

A different approach would be to use Perl as a server-side language on Postgres. This page tells you how: http://www.oreillynet.com/pub/a/databases/2005/11/10/using-perl-in-postgresql.html

Their example is:

postgres$ createlang plperlu mydb
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A Simple Example

The easy way to show how to use PL/Perl is to create a very simple function; one that would be a lot harder to do otherwise. Suppose that you want to test if a given piece of text is a palindrome (a word that reads the same backwards as forwards), disregarding white space and the case of the letters. Here's a piece of SQL to define the function:

create function palindrome(text) returns boolean 
language plperl 
immutable
as '

  my $arg = shift;
  my ($canonical = lc $arg) =~ s/\s+//g;
  return ($canonical eq reverse $canonical) ? "true" : "false";

';
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Given this function, you can write SQL like:

select name, region, country, population
from towns
where palindrome(name);
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