use strict;
use warnings;

my $a = "123";
foo($a, "456");

sub foo {
    for (my $i = 0; $i < @_; $i++) {
        my $type = eval { $_[$i] = "variable" } || "constant";
        printf "Arg #%d is $type\n", $i + 1;
    }
}

##</code><code>##

Arg #1 is variable
Arg #2 is constant