good point, $1 sticks to the last successful match

so always check before accessing it

for (qw(a X b)) {
    my $id = $1
      if /(a)/ or /(b)/;

   print qq{'$_' -> '$id'\n};

}
[download]

C:/Perl_524/bin\perl.exe -w d:/exp/pm_or_regex.pl 
Use of uninitialized value $id in concatenation (.) or string at d:/ex
+p/pm_or_regex.pl line 5.
'a' -> 'a'
'X' -> ''
'b' -> 'b'
[download]

I kept the warning in intentionally.