glob will do it, if you transmogrify the pattern to its liking.

my $str1 = "ATC[TG]C[CA]";

(my $glob_pat = $str1) =~ s!\[(.*?)\]! '{' . join(',',split//,$1) . '}
+'!ge;
print "Pattern is $glob_pat\n";
use Data::Dumper;
print Dumper([glob($glob_pat)]);
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Actually, character classes as you give are used by glob, but it looks for files with those names. When you use curly-brace alternation, it doesn't require that there be files with those names.