This is the "spaceship" operator for comparing in a numeric sense:
#!/usr/bin/perl -w
use strict;
my $x = 2;
my $y = 3;
my $cmp_result = $x <=> $y;
print $cmp_result # -1
Note: while naming variables the same as Perl key words is allowed (e.g. $cmp, (cmp is a key word), I personally try to avoid this).
for comparison in a string context, use cmp:
my $cmp_result = $stringX cmp $stringY;
Update: As to the history, I have no idea why <=> is called the "spaceship" operator, but that is the common usage.