I am, as an exercise, trying to write a function that mimics pattern matching in langauges like Haskell and OCaml in Perl. The following code runs (I'm using Perl 5.18) and produces the following result. 
$ perl pattern_matching.pl 
Name "main::bob" used only once: possible typo at pattern_matching.pl 
+line 50.
Use of uninitialized value $bob in print at pattern_matching.pl line 5
+0.

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I think that's because the while loop 
while (my ($k, $v) = each %$locals) {
    local ${$k} = $v;
}

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localizes each variable only within the body of the loop. I'm wondering what the right approach would be. Entire code below. 
use strict;
use warnings;

sub same_keys {
    my ($left, $right) = @_;
    return unless ref $left eq 'HASH' and ref $right eq 'HASH';
    return unless keys %$left == keys %$right;
    for my $key (keys %$left) {
        exists $right->{$key} or return;
    }
    return 1;
}

sub match ($$$) {
    no strict 'refs';
    my ($lhs, $rhs, $fun) = @_;
    my $locals = {};
    # populate locals
    match_inner($lhs, $rhs, $locals);

    while (my ($k, $v) = each %$locals) {
        local ${$k} = $v;
    }
    return $fun->();
}

sub match_inner {
    my ($lhs, $rhs, $locals) = @_;
    if (ref $lhs eq '') {
        $locals->{$lhs} = $rhs;
    }
    elsif (ref $lhs eq 'ARRAY') {
        if (@$lhs == @$rhs) {
            for my $i (0 .. $#$lhs) {
                match_inner($lhs->[$i], $rhs->[$i], $locals);
            }
        }
    }
    elsif (ref $lhs eq 'HASH') {
        if (same_keys($lhs,$rhs)) {
            for my $k (keys %$lhs) {
                match_inner($lhs->{$k}, $rhs->{$k}, $locals);
            }
        }
    }
}

match "bob", 45, sub {
    no strict;
    print $bob;
};

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In reply to rough approximation to pattern matching using local by gregory-nisbet

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