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
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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;
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Update: As to the history, I have no idea why <=> is called the "spaceship" operator, but that is the common usage.