no I didn't, it was just a guess :-) and in fact, it seems the double reverse solution is faster for small strings, while the substr solution wins in the long run. anyway, here you get full benchmark:

use strict;
use List::Util qw(shuffle);
use Benchmark qw(:all);

my $string = "ACta";

print "*** ", length($string), " characters\n";
cmpthese(1_000_000, {
    uclast_substr => sub { uclast_substr($string) },
    uclast_2reverse => sub { uclast_2reverse($string) },
});

$string = join ("", shuffle( 'A'..'Z', 'a'..'z' ));

print "*** ", length($string), " characters\n";
cmpthese(1_000_000, {
    uclast_substr => sub { uclast_substr($string) },
    uclast_2reverse => sub { uclast_2reverse($string) },
});

$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));

print "*** ", length($string), " characters\n";
cmpthese(1_000_000, {
    uclast_substr => sub { uclast_substr($string) },
    uclast_2reverse => sub { uclast_2reverse($string) },
});

$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));
$string .= join ("", shuffle( 'A'..'Z', 'a'..'z' ));

print "*** ", length($string), " characters\n";
cmpthese(1_000_000, {
    uclast_substr => sub { uclast_substr($string) },
    uclast_2reverse => sub { uclast_2reverse($string) },
});


sub uclast_substr {
    my($string) = @_;
    $string = lc(substr($string, 0, length($string)-1)) 
            . uc(substr($string, -1, 1));
    return $string;
}

sub uclast_2reverse {
    my($string) = @_;
    $string = reverse ucfirst lc reverse $string;
    return $string;
}
[download]

and here the result:

*** 4 characters
                    Rate   uclast_substr uclast_2reverse
uclast_substr   566572/s              --            -15%
uclast_2reverse 666223/s             18%              --
*** 52 characters
                    Rate   uclast_substr uclast_2reverse
uclast_substr   470588/s              --             -6%
uclast_2reverse 500000/s              6%              --
*** 208 characters
                    Rate uclast_2reverse   uclast_substr
uclast_2reverse 249938/s              --             -9%
uclast_substr   273523/s              9%              --
*** 676 characters
                    Rate uclast_2reverse   uclast_substr
uclast_2reverse 102239/s              --            -19%
uclast_substr   126231/s             23%              --