my @strings = qw( 1234579 234789 4578 );

my %c = map { $_ => 1 } split //, shift @strings;
%c = map { $_ => 1 } grep { $c{$_} } split // for @strings;

print keys %c, "\n";
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This will give you a list of characters, but it doesn't preserve order. When I ran it, I got '47', but it could just as easily have been '74'. I can't tell from your problem definition whether that's relevant or not, but I suspect it's not.