Benchmark these two and pick your favorite. substr($a,0,36) eq substr($b,0,36) or unpack('A36', $a) eq unpack('A36', $b). If those aren't fast enough then consider implementing your compare function in C and call out to it using Inline::C.

How would eq efficiently do this

use Inline C;
use POSIX     qw( strncmp );
use Benchmark qw( cmpthese );

use strict;
use warnings;

my $n       = 36;
my @nums    = 0 .. 99_999;
my @str     = map join('', 'a' .. 'z'), @nums;

no warnings qw/ uninitialized void /;

cmpthese(-10, {
    strncmp   => sub {
        for(@nums) {
            strncmp_perl(@str[$_,$_ + 1], $n);
        }
    },
    eq_substr => sub {
        for(@nums) {
            substr($str[$_], 0, $n) eq substr($str[$_ + 1], 0, $n)
        }
    },
    eq_unpack => sub {
        for(@nums) {
            unpack("A$n", $str[$_]) eq unpack("A$n", $str[$_ + 1])
        }
    },
});

__END__
__C__

int strncmp_perl(char* s1, char* s2, int n) {
    return strncmp(s1, s2, (int)n);
}

__output__

Benchmark: running eq_substr, eq_unpack, strncmp, each for at least 10
+ CPU seconds...
 eq_substr: 11 wallclock secs (10.11 usr +  0.01 sys = 10.12 CPU) @  5
+.34/s (n=54)
 eq_unpack: 11 wallclock secs (10.16 usr +  0.00 sys = 10.16 CPU) @  2
+.07/s (n=21)
   strncmp: 11 wallclock secs (10.04 usr +  0.00 sys = 10.04 CPU) @  5
+.88/s (n=59)
            Rate eq_unpack eq_substr   strncmp
eq_unpack 2.07/s        --      -61%      -65%
eq_substr 5.34/s      158%        --       -9%
strncmp   5.88/s      184%       10%        --
[download]

_________
broquaint

Thanks for that. I used the following comparison in a benchmark:

index(substr($str[$_], 0, $n), $str[$_ + 1], 0) != 0
[download]

and came up with:

Benchmark: running eq_index, eq_substr, each for at least 10 CPU secon
+ds...
  eq_index: 14 wallclock secs (10.08 usr +  0.00 sys = 10.08 CPU) @  3
+.08/s (n=31)
 eq_substr: 16 wallclock secs (10.30 usr +  0.00 sys = 10.30 CPU) @  3
+.01/s (n=31)
            Rate eq_substr  eq_index
eq_substr 3.01/s        --       -2%
eq_index  3.08/s        2%        --
[download]

Thoughts or comments?

-Lynn

update (broquaint): added formatting

You want to use eq for checking the equality of strings.

perlop says:

Binary ``=='' returns true if the left argument is numerically equal to the right argument.
Binary ``eq'' returns true if the left argument is stringwise equal to the right argument.

Update! The docs for Posix say this:

strcmp() is C-specific, use eq or cmp instead, see the perlop manpage.

2nd update diotalevi has a better answer since I didn't take into account that you only want to compare the first 36 characters.