Ah yes, that would be

sub is_numeric {
  ($_[0] & ~ $_[0]) eq "0";
}
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Which tests whether a scalar is a numeric value or a string value e.g 42 vs "42". Whereas the snippet discerns whether a given scalar looks like a number or not e.g

print +(isnum($_) ? "is a number" : "not a number"), ": $_", $/
  for qw( 1 2.2 3e3 4. .5);

__output__

is a number: 1
is a number: 2.2
is a number: 3e3
is a number: 4.
is a number: .5
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HTH

_________
broquaint