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for (qw(a X b)) {
    my $id = $1
...
   print qq{'$_' -> '$id'\n};

}

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'