How would eq efficiently do this

While I'm always a little dubious of benchmarks but here's one for your given case

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]

So it looks like the Inline::C strncmp() is slightly faster than substr() and eq combined, but always take benchmarks with a grain of salt.
HTH

_________
broquaint