That is a very interesting question!

I don't know if this is the best answer, but my first thought was to try assigning to the variable and see what happens:

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;
    }
}
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The output of this is:

Arg #1 is variable
Arg #2 is constant
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Since the write operation is wrapped in an eval, it returns undef if the assignment failed (constant), otherwise the result of the assignment.