Someone looking at your example could think that use bigint alters how many digits print outputs, but that would be wrong. Here's a better example:

{
   $a = 2**59;        # Saved as a limited precision float.
   $b = 2**59 + 1;    # Saved as a limited precision float.
   print($a,    $/);  # Prints "5.76460752303423e+017".
   print($b,    $/);  # Prints "5.76460752303423e+017".
   print($b-$a, $/);  # Prints "0".
}

{
   use bigint;
   $a = 2**59;        # Saved as an unlimited precision int.
   $b = 2**59 + 1;    # Saved as an unlimited precision int.
   print($a,    $/);  # Prints "576460752303423488".
   print($b,    $/);  # Prints "576460752303423489".
   print($b-$a, $/);  # Prints "1".
}
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