You must be using a pre-5.10 version of Perl. Try:

#!/usr/bin/perl -slw
use strict;
use Data::Dump qw[ pp ];

our $Y ||= 10; ## changed from //= for pre-5.10 
our $X ||= 10;

my @xs = map -3+rand 6, 1 .. $X;
my @ys = map -3+rand 6, 1 .. $Y;

my %bins;

for my $x ( @xs ) {
    my $xc = ( int( $x * 10 )  + 0.5 ) / 10;
    for my $y ( @ys ) {
        my $yc = ( int( $y * 10 ) + 0.5 ) / 10;
        ++$bins{ "$xc:$yc" };
    }
}

pp \%bins;

printf "Buckets used: %d, total values %d \n",
    scalar keys %bins, sum values %bins;
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You can vary the number of datapoints generated by using a command line like:

perl -s theScript.pl -X=100 -Y=100
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