use Set::CrossProduct;

my $str = "ATC[TG]CC";
my $strings = [];

my @wildcard;

$str =~ s{ \[ ([ATCG]+) \] }{ push @wildcard, $1; "%s"; }xgem;

# now your string looks like "ATC%sCC"

if( @wildcard ) {
    my @set = map [ split //, $_ ], @wildcard;

    # now @set contains ( [ 'T', 'G' ] )
    # and we weave each possible combination into the %s placeholders in $str

    my $xp = Set::CrossProduct->new( \@set );
    while( my @tuple = $xp->get ) {
        push @$strings, sprintf $str, @tuple;
    }
}
else {
    push @$strings, $str;
}