well, can use eval:

my $combined = join ' ', grep $_, map { my $value = eval "\$$_"; $valu
+e ? sprintf("%s=%s", $_, $value) :  undef } ( 'a' .. 'z' );
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But comes with all the caveats of eval'ing arbitrary variables. I hope someone posted a way of doing this, which would make the map a lot simpler:

  my $x = 3;
  print f($x);  # 'x'
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Also note that it is bad to use $a and $b becaues it can/will cause confusion with the special variables used by sort.