7.7.1. The "e" Switch

If an e is appended to the end of the substitution command, then the right side is treated as a normal Perl expression, giving you the ability to use operators and functions.

The following example, replaces the first word in the sentence with its length:

use strict;
use warnings;

my $string = shift;

$string =~ s/^([A-Za-z]+)/length($1)/e;

print $string, "\n";

And the following example, converts the first word that starts with the letter "S" into uppercase:

use strict;
use warnings;

my $string = shift;

$string =~ s/([Ss][A-Za-z]*)/'<'.uc($1).'>'/e;

print $string, "\n";