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

##</code><code>##

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";

##</code><code>##

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";