>perl -wMstrict -le
"my ($w, $x, $y) = qw(YUU EKS);
 ;;
 my $scalar = qr{ \$ \s* (?: { \s* \w+ } | \w+ ) }xms;
 ;;
 my $interpol = q{hi from '$w' and ${x}also $y and $z too};
 ;;
 $interpol =~ s{ ($scalar) }
               { no warnings qw(ambiguous);
                 my $e = eval qq{$1};
                 $@            ? qq{[['$1' non-existant]]} :
                 ! defined($e) ? qq{[['$1' undefined]]}    :
                                 $e;
               }xmseg;
 ;;
 print qq{'$interpol'};
"
'hi from 'YUU' and EKSalso [['$y' undefined]] and [['$z' non-existant]] too'