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]

*** 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%              --

use Benchmark;

my $tests = [qw(these are a few test to try
                andaverylongteststring
                acoupleoftimesover
                xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxx
                zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
+zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
zzzzzzzzzzzzzzzzzzzzzzzzzzzzz)];

timethese (100000, {
NoOpLoop =>
sub {
  for my $test (@$tests) {
    # NO Op Loop
  }
},
regex1 =>
sub {
  for my $test (@$tests) {
    $test=~s/^(.*)(.)$/\L$1\U$2/;
  }
},
substr =>
sub {
  for my $test (@$tests) {
    $test= lc(substr($test, 0, length($test)-1))
           . uc(substr($test, -1, 1));
  }
},
reverse=>
sub {
  for my $test (@$tests) {
    $test=reverse ucfirst lc reverse $test;
  }
}
});
[download]

Benchmark: timing 100000 iterations of NoOpLoop, regex1, reverse, subs
+tr...
  NoOpLoop:  2 wallclock secs ( 1.86 usr +  0.00 sys =  1.86 CPU) 
             @ 53763.44/s (n=100000)
    regex1: 82 wallclock secs (30.31 usr +  0.01 sys = 30.32 CPU) 
             @ 3298.15/s (n=100000)
   reverse:  8 wallclock secs ( 7.40 usr +  0.00 sys =  7.40 CPU) 
             @ 13513.51/s (n=100000)
    substr:  8 wallclock secs ( 8.51 usr +  0.01 sys =  8.52 CPU) 
             @ 11737.09/s (n=100000)
[download]

Hope it helps
UnderMine