Inspired by previous answers. As usual, the absence of bugs is totally not guaranteed :) But should be something like that... 
use strict;
use warnings;

# greatest common multiple
sub gcm {
    my ( $max, $num ) = @_;

    for ( my $c = 1; ( $num & 1 ) == 0 && $c < 4096; $c <<= 1 ) {
        $num >>= 1;
    }
    my $lcm = 4096 * $num;    # least common multiple

    return int( $max / $lcm ) * $lcm;
}

sub pretty {
    return shift =~ s/\d \K (?= (?: \d{3} )+ \z)/_/xgr;
}

print pretty( gcm( 2_147_483_648, 12 ) ),    "\n";
print pretty( gcm( 3_221_225_472, 5007 ) ),  "\n";
print pretty( gcm( 4_096,         4_096 ) ), "\n";
print pretty( gcm( 8_192,         8_192 ) ), "\n";
print pretty( gcm( 15_000,        8_192 ) ), "\n";
print pretty( gcm( 17_000,        8_192 ) ), "\n";
print pretty( gcm( 17_000,        10 ) ),    "\n";

[download]
output: 
2_147_475_456
3_219_861_504
4_096
8_192
8_192
16_384
0

[download]

In reply to Re: Simple arithmetic? by Anonymous Monk
in thread Simple arithmetic? by BrowserUk

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