How to pass arrays to subroutines without flattening in Perl?

In Perl, subroutine arguments are always received as a single flat list in @_. When you pass multiple arrays to a subroutine directly, Perl concatenates them, losing the boundaries between arrays. The standard solution is to pass references to arrays instead of the arrays themselves.

Why Arrays Flatten

When you call a subroutine with multiple arrays, Perl flattens them into one list. Inside the subroutine, @_ contains all elements in sequence with no way to distinguish where one array ends and another begins.

Solution: Pass Array References

By passing references using the backslash operator \@array, you provide scalar pointers to the arrays. The subroutine receives distinct scalar references that can be dereferenced to access the original arrays.

Complete Runnable Example

#!/usr/bin/perl
use strict;
use warnings;

# Subroutine expects two array references
sub print_arrays {
    my ($arr1_ref, $arr2_ref) = @_;
    
    # Dereference to access array elements
    print "Array 1: ", join(", ", @{$arr1_ref}), "\n";
    print "Array 2: ", join(", ", @{$arr2_ref}), "\n";
}

my @array1 = (1, 2, 3);
my @array2 = ('a', 'b', 'c');

# Pass references to avoid flattening
print_arrays(\@array1, \@array2);

Key Concepts

• \@array creates a reference (scalar) to an array
• @{$ref} dereferences back to the array
• References preserve array boundaries since they're scalar values
• Works in all Perl 5 versions

Common Pitfalls

• Trying my (@a, @b) = @_; doesn't work—@a consumes all of @_ in list context
• Forgetting to dereference prints memory addresses instead of contents
• Passing arrays directly loses the ability to distinguish between them

This reference-based approach is the idiomatic Perl solution (part of TMTOWTDI) and is essential for writing maintainable code that handles multiple arrays cleanly.