Perhaps you need to be a bit more specific on the definition of your matching patterns. You said first five letters are any letters in @group1, should it be rephrased like first five letters should be any letters in @group1, but can contain other letters not in @group2? Otherwise I see that your example works well for patterns with first five characters only from @group1 and next two characters only from @group2.

@group1 = ('A','B','C','D');
@group2 = ('E','F','G','H');
$string='ABDCDEFABDECAFBBCDAGH';
while ($string =~ /([@group1]{5}[@group2]{2})/g)
{
    print "$1\n";
}
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And the output:

ABDCDEF
BBCDAGH
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