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use warnings; 
use strict; 
...
        while (0 != $a) { $n = int($b / $a); ($a, $b, $x, $y) = ($b - 
+$n * $a, $a, $y - $n * $x, $x); } 
        $y % $m; 
}

# WRONG, DO NOT USE
use warnings; 
...
        while (0 != $a) { $n = int($b / $a); ($a, $b, $x, $y) = ($b - 
+$n * $a, $a, $y - $n * $x, $x); } 
        $y % $m; 
}