The URI::Escape module fruiture mentioned will do exactly what you are looking for.

Here is essentially what that module would do, and how you can do the same without it.

# To encode 
$string =~ s/([^a-zA-Z0-9_\-.])/uc sprintf("%%%02x",ord($1))/eg;

# To decode
$string =~ s/%([0-9A-Fa-f]{2})/chr(hex($1))/eg;
[download]

It is always nice to know how stuff works, but it is almost always
better to still use the already thought through, (hopefully mostly debugged :-))
solutions found in the modules on CPAN.

Wonko