Try this, but note that you should take care of any key presses unless you run this in a window environment (e.g. Win32::GUI):

#!/usr/bin/perl -w
use strict;
use Win32::API;


use constant VK_SHIFT => 0x10;
use constant VK_CONTROL => 0x11;
use constant VK_MENU => 0x12;


=head2 GetAsyncKeyState($keyCode)

Retrieve the status of the specified virtual key. The status 
specifies whether the key is up, down, or toggled (on, off--
alternating each time the key is pressed). 

$keyCode -- A single character string: A..Z0..9, or a 
virtual key code. Example: VK_SHIFT, "C"

Return 1 if the key is depressed, 0 if it's not.

=cut
my $rsGetAsyncKeyState = new Win32::API("user32", "GetAsyncKeyState", 
+"I", "N");
sub GetAsyncKeyState {
    my ($keyCode) = @_;    
    $keyCode = ord($keyCode) if($keyCode =~ /^[A-Z0-9]$/);

    my $ret = $rsGetAsyncKeyState->Call($keyCode);
    $ret = unpack("S", pack("s", $ret));
    
    return( $ret & 2**15 ? 1 : 0 );
    }


my $key = VK_SHIFT;
#my $key = "8";
#my $key = "J";
while(1) {
    print "state: " . GetAsyncKeyState($key) . "\n";
    sleep(1);
    }
[download]

There is more to read about this if you look up GetAsyncKeyState in the Win32 API, e.g. here: http://www.sxlist.com/techref/os/win/api/win32/func/src/f27_3.htm

/J