You don't need an if statement because the expression

$_[0] =~ /^\s*\+\-?\d+\s*$/
[download]

is evaluated before the return.

In a scalar context it returns the number of matches.

Therefore as the regex is anchored start and end, if the supplied string is an integer the regex returns 1 otherwise 0, which are evaluated as true and false. Thus the above routine could be employed as follows

if (is_integer_string($input){
   do_integer_function($input);
}
else{
   die "You must supply an integer\n";
}
[download]

You need to check if the modulus of the numbers is zero, perhaps a has_remainder function?

Hope that clarifies.