#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %hash = (
	'f_foo' => 'bleh',
	'f_bar' => 'blarf',
	'f_bas' => 'heh',
);

foreach(keys %hash) {
  $hash{$1} = $hash{$_} if(m/f_(.*)/);
}

print Dumper(\%hash);