Build a list of references to the hashes. Use map to build a key count. Use grep to find duplicate keys:

use strict;
use warnings;

# The hashes
my %hash1 = (1..6);
my %hash2 = (1..4, 7..10);
my %hash3 = (1, 2, 9..12);

# Build the list of hash references
my @hashList = (\%hash1, \%hash2, \%hash3);
my %keyCount;

# Count the keys
map {$keyCount{$_}++} keys %$_ for @hashList;

# Find duplicates
my @common = sort grep {$keyCount{$_} == @hashList} keys %keyCount;

print "@common";
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Prints:

1
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