Is there a way to initialize local variables (e.g. $k for 'key', and $v for 'value') and then have these loaded into the scope of the called code? e.g. like sort does with "a" and "b".

Yes and no. Yes, you can set variables in the caller's package (like sort does with $a and $b).

sub hmap(&@) {
   my $cb = shift;

   my $caller = caller();
   my $kgr = do { no strict 'refs'; \*{$caller.'::k'} };
   my $vgr = do { no strict 'refs'; \*{$caller.'::v'} };
   local *$kgr = \my $k;
   local *$vgr = \my $v;

   for (...) {
      $k = ...;
      $v = ...;
      $cb->();
   }
}
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But while $a and $b are exempt from strict, $k and $v aren't. You'd have to use something like

... = hmap { our($k,$v); ... } ...;
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So there isn't really an advantage over using arguments as you are doing now.

... = hmap { my($k,$v)=@_; ... } ...;
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One possible workaround is to use $a and $b instead of $k and $v. That's what List::Util does for reduce.

Another possible workaround is to export $k and $v from the module that provides $hmap. Imported variables are exempt from strict.

package Hash::Map;

use Exporter qw( import );
our @EXPORT = qw( $k $v hmap hgrep );

sub hmap(&@)  { ... }
sub hgrep(&@) { ... }

1;
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