Update: figured it out (simulthanks to Ikegami). Gotta reference the right package. Corrected solution: 
sub map_pairs(&@) {
  my $fn = shift;

  my $pkg = caller;

  map {
    my $idx = $_ * 2;
    no strict 'refs';
    my ($a, $b)
    = local (${$pkg.'::a'}, ${$pkg.'::b'})
    = (@_[$idx,$idx+1]);
    $fn->($a, $b);
  } (0..$#_/2);
}

[download]
Previous, erroneous solution follows.

I think you made this harder than it needs to be. Isn't this equivalent? 

sub map_pairs(&@) {
  my $fn = shift;
  map {
    my $idx = $_ * 2;
    local ($a, $b) = @_[$idx,$idx+1];
    $fn->($a, $b);
  } (0..$#_/2);
}

[download]
Caution: Contents may have been coded under pressure.

In reply to Re^2: Mini-Tutorial: Working with Odd/Even Elements by Roy Johnson
in thread Mini-Tutorial: Working with Odd/Even Elements by ikegami

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