My solution is not really a competitor, since I'm not that familiar with Perl Golf customs, so bear with me.

use strict;

sub f {
  my ($a,$b,$m,$n) = @_;
  my (@r) = ();
 
  my $l = " $a! $b";
  while($l =~ s/(.*?)(( \w+){$m,$n} )(.*!.*\2.*)/ $1 =$4/) {
    push @r, $2;
  };
  @r
};
[download]

Why I'm posting this at all is, that a previous version exhibited unanticipated (by me, that is) behaviour :

  # Previous version :
  my $l = " $a! $b";
  while($l =~ s/(.*)(( \w+){$m,$n} )(.*!.*\2.*)/ $1 =$4/) {
    push @r, $2;
  };
  @r
[download]

That version only found the $m-word matches, because the greedy (.*) at the beginning forced the RE engine to work its way backwards through the string. As soon as I sacrificed another character, this problem went away. I haven't given the golfing much thought (at work, I can only think about concepts, not byte-fiddle with REs without raising suspicions ... ).