I wrote basically the same solution, but like having the loop variable (esp. since there's a use of $_ in the map.)

We both changed the conditional in different ways, from ne '' in the original to evaluates true in yours, and to existence in mine.

for my $hashref (@$sur) {
  if (exists $hash1{$hashref->{three}}) {
    $hash1{$hashref->{three}} .= "####$hashref->{four}";
  }
  else {
    $hash1{$hashref->{three}} = join '<-->', map $hashref->{$_}, (qw(o
+ne two four));
  }
}
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