If the question actually means "to match any character within a group of characters", not "a sequence of characters" (it at least sounds not very clear to me), then you may want to use [], see the following example:

my $line = "abd";
if ($line =~ /abcd/i) {
    print "matched the first case\n";
} 
if ($line =~ /[abcd]/i) {
    print "matched the second case\n";
}
[download]