You may be able to exploit the ~ operator, it has deep knowledge of Perl's internals. For example:

$string = "1";
$integer = 1;

print ~$string;  # Prints an extended character
print ~$integer; # Prints 4294967294
[download]

Much easier than messing with perlguts. But there's one caveat: using ~ on a string with extended characters results in ASCII characters; that is ~chr(205) is "2". Assuming your definition of "string" does not include extended characters (only ASCII), you should be able to get away with a function like this:

sub is_integer
{
    return (~$_[0] =~ m/^[0-9]+$/) ? 1 : 0;
}

print "1        ", is_integer(1),       "\n";   # 1 ok
print "'1'      ", is_integer('1'),     "\n";   # 0 ok
print "chr(205) ", is_integer(chr(205)),"\n";   # 1 not ok
[download]

That said, be careful with this power. You don't want to confuse your users. Problems occur when reading data from files or console, as everything read is a string unless explically changed via +0. This also doesn't work for floating point. But sometimes it may have useful uses.

Hope this helps,

Jeff Connelly

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