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my @array_of_test_names = (
    "History--abcdef",
...
    "History--123",
    "History--1X",
);

my $mask = '';
$mask ^= $_ ^ $array_of_test_names[0] for @array_of_test_names;
$mask =~ m[^(\0+)] and $len = length( $1 );
print substr $array_of_test_names[ 0 ], 0, $len;
print "\n";

my $res = '';
for my $i ( 0 .. (length($array_of_test_names[0]) -1)) {
...
    $res .= $letter;
};
print $res,"\n";