But that doesn't work for arrays with odd numbers of elements:

>perl -wMstrict -MData::Dump -le
"my @array_of_test_names = qw(ABCxyz  ABCDEfoo  ABCDbar);
 ;;
 my $mask = '';;
 $mask ^= $_ for @array_of_test_names;;
 dd $mask;
 ;;
 my $len;
 $mask =~ m[^(\0+)] and $len = length( $1 );;
 my $min = substr $array_of_test_names[ 0 ], 0, $len;;
 print qq{'$min'};
"
"ABCx^}\35o"

Use of uninitialized value $len in substr at -e line 1.
''
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Update: I didn't see mbethke's post until after I posted, but I agree that simple fix will solve the odd-element problem.