It's also ambiguous if all matches fail: 

c:\@Work\Perl\monks>perl -wMstrict -le
"for (qw(a X b)) {
   /(a)/ or /(b)/;
   my $id = $1;
   print qq{'$_' -> '$id'};
   }
"
'a' -> 'a'
'X' -> 'a'
'b' -> 'b'

[download]

Update 1: Something like
    my $id = $1 // $2 if m!/key/(\d+)|/(\d+)-\d+!;
(if you avoid the problem of the conditionally declared lexical!) does not suffer this: 

c:\@Work\Perl\monks>perl -wMstrict -le
"for (qw(a X b)) {
   /(a)|(b)/ or next;
   my $id = $1 // $2;
   print qq{'$_' -> '$id'};
   }
"
'a' -> 'a'
'b' -> 'b'

[download]

Update 2: Slight wording change to first sentence.

Give a man a fish:  <%-{-{-{-<

In reply to Re^2: capture value from two patterns (updated) by AnomalousMonk
in thread capture value from two patterns by vitoco

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