$ cat destructive_sub.pl
use warnings;
use strict;

my $param = 'Now is the time';

print "$param\n";
do_it($param);
print "$param\n";

sub do_it {
    $_[0] =~ s/([it])/uc($1)/ge;
}

krevulax:~ [roboticus]
$ perl destructive_sub.pl
Now is the time
Now Is The TIme