Perhaps you are trying to see which of the two alternations matched. I wonder if this would be close enough.

$ perl -E '
> $rx = qr{(?x)
>    (
>    foo\d+(?{say q{matched foo\d+}})
>    |
>    bar\S+?(?{say qq{matched bar\S+?}})
>    )
>    };
> q{abcfoo44fred} =~ $rx && say qq{captured $1};
> q{barbell} =~ $rx && say qq{captured $1};
> q{xyz} =~ $rx && say qq{captured $1};'
matched foo\d+
captured foo44
matched bar\S+?
captured barb

$
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I hope this is helpful.