Not sure if I miss something, but it seems there is a problem.

With this array for example:

my @array_of_test_names = (
    "History--abcdef",
    "History--aaa",
    "History--123",
    "History--1X",
);
[download]

updated: bad try

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";
[download]

update2: btw, a not pretty solution

my $res = '';
for my $i ( 0 .. (length($array_of_test_names[0]) -1)) {
    my $letter = substr ($array_of_test_names[0],$i,1);
    last if ( scalar grep { $letter ne substr($array_of_test_names[$_]
+,$i,1)} (0 .. $#array_of_test_names));
    $res .= $letter;
};
print $res,"\n";
[download]