sub use_substr {
        my $string = shift;
        $string .= substr($string, -1) eq "/" ? "" : "/";
        return $string;
}
sub use_if_substr {
        my $string = shift;
        $string .= '/' if substr($string, -1) ne "/";
        return $string;
}

sub use_if_regex {
        my $string = shift;
        $string .="/" if(not $string =~/\/$/ );
        return $string;
}

sub use_regex {
        my $string = shift;
        $string =~ s|/?$|/|;
        return $string;
}

use Benchmark 'cmpthese';

my @strings = ( qw( a / sdf/ /some/path /another/path/ sdfasdfasdfsdaf
+sadfasdfsfdsadfsdfsdfsadfasdfsadfsadfsadfsadfsdfsadfasfsdafasdfsdf ),
+ 'some string with spaces in it' );

cmpthese ( 100000, {
        'if_substr' => sub { use_if_substr($_) for @strings },
        'substr'    => sub { use_substr($_)    for @strings },
        'if_regex'  => sub { use_if_regex($_)  for @strings },
        'regex'     => sub { use_regex($_)     for @strings }
} );
[download]

Results in:

Benchmark: timing 100000 iterations of if_regex, if_substr, regex, sub
+str...
  if_regex:  3 wallclock secs ( 2.50 usr +  0.00 sys =  2.50 CPU) @ 40
+000.00/s (n=100000)
 if_substr:  2 wallclock secs ( 2.47 usr +  0.00 sys =  2.47 CPU) @ 40
+485.83/s (n=100000)
     regex:  5 wallclock secs ( 4.88 usr +  0.00 sys =  4.88 CPU) @ 20
+491.80/s (n=100000)
    substr:  4 wallclock secs ( 2.57 usr +  0.00 sys =  2.57 CPU) @ 38
+910.51/s (n=100000)
             Rate     regex    substr  if_regex if_substr
regex     20492/s        --      -47%      -49%      -49%
substr    38911/s       90%        --       -3%       -4%
if_regex  40000/s       95%        3%        --       -1%
if_substr 40486/s       98%        4%        1%        --
[download]