You could also use Math::BigInt:

#!/usr/bin/perl

use Math::BigInt;

my $hex = "0x000fffff7f00fffc";
my $dec = Math::BigInt->new($hex);

print "$hex = $dec\n";

# also works with even larger numbers...

$hex = "0x000fffff7f00fffc000fffff7f00fffc000fffff7f00fffc000fffff7f00
+fffc";
$dec = Math::BigInt->new($hex);

print "$hex = $dec\n";

__END__
0x000fffff7f00fffc = 4503597463175164
0x000fffff7f00fffc000fffff7f00fffc000fffff7f00fffc000fffff7f00fffc = 2
+826953945157987506191153074870519081847080605839924685569054791495483
+3916
[download]

Update: maybe it's worth noting that $dec is treated as a number internally (as opposed to a simple string representation of it), i.e. you can calculate with it. The respective operators are overloaded.