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]