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sub is_numeric {
  ($_[0] & ~ $_[0]) eq "0";
}

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