I can't replicate the problem here. How about simplifying the algorithm instead?

sub minutes_before
{
    my $minutes_b4=shift;
    my $month=shift;
    my $day=shift;
    my $hour=shift;
    my $minute=shift;

    # Lets find n minutes before!
    my $datetime=DateTime->new( year   => 2005,
                             month  => month_to_num($month),
                             day    => $day,
                             hour   => $hour,
                             minute => $minute,
                             second => 00 );

    $datetime->subtract( minutes => $minutes_b4 );

    $ready=1;
    return @MINBEFORE = (
        num_to_month( $datetime->month ),
        $datetime->day,
        $datetime->hour,
        $datetime->minute );
}
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