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sub file_merge {
  my $hash_ref = shift;
...
    push @{$hash_ref->{$key}}, [ @values ];
  }
}

sub file_merge {
  my $filename = shift;
...

  return %hash;
}

my %msisdn = file_merge('/tmp/subscription_list');
my %calls  = file_merge('/tmp/Call_Det');