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 k1   v1 k2   v2 k3   v1     v1           v2
(a => 1, b => 2, c => 1) => (1  => [a,c], 2 => [b])

use strict;
use warnings;
...
    push @{$inverted{$hash{$_}}}, $_ for keys %hash;
    return map {@{$inverted{$_}} > 1 ? @{$inverted{$_}} : ()} keys %in
+verted;
}

a
c