as long as all the values in %hash are unique (or if you don't mind loosing keys that have repeated values), you can use:

my %rhash = reverse %hash;
my $key = $rhash{$value};
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and if you have repeated values:

my %rhash;
for (keys %hash) {
  no warnings 'uninitialized';
  push @{$rhash{$hash{$_}}}, $_;
}

print("$value keys: ", 
      join(', ', @{$rhash{$value}}),
      "\n");
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