use 5.010;
use Scalar::Util qw(looks_like_number);

sub is_int (_) {
  my $i = shift;
  looks_like_number($i) and int($i)==$i;
}

say for grep { is_int } qw(
    -6
    5.2
    5.0
    3e4
    0Hello
);
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Note that this classes 5.0 as an integer, which is correct from a maths point of view, but not necessarily from a comp sci one.

I was thinking of something along the lines of the following:

  print TQIS::Primitive::typeis("1") ;
> string
  print TQIS::Primitive::typeis(1) ;
> integer
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Here is the code:

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

SV* typeis ( SV* what ) ;

SV* typeis ( SV* what )
{
        if ( SvIOK( what ) )
                return newSVpvs( "integer" ) ;
        else if ( SvNOK( what ) )
                return newSVpvs( "double" ) ;
        else if ( SvPOK( what ) )
                return newSVpvs( "string" ) ;

        return newSVpvs( "unknown" ) ;
}


MODULE = TQIS::Primitive                PACKAGE = TQIS::Primitive

PROTOTYPES: ENABLE

SV*
typeis( what )
        SV* what
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Here's another option using Regexp::Common:

use strict;
use warnings;
use Regexp::Common qw/ number /;

print for grep /^$RE{num}{int}$/, <DATA>;

__DATA__
3.14
10
-42
42
.14159
12.34e-56
3.0
Hello, world.
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Output:

10
-42
42
[download]

How about Scalar::Util::Numeric?

This module exports a number of wrappers around perl's builtin grok_number function,...