>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'

[download]

Update: I like this one better: no no warnings, and it supports  \$ escaped dollars and all forms of scalars I know of, which are normalized to a canonical form (sorry for any pesky wraparound, which I tried to fix this up a bit) (tested under 5.8.9 and 5.14.2): 

>perl -wMstrict -le
"my ($w, $x, $y) = qw(YUU EKS);
 ;;
 my $interpol
   = q{hi \$w from '$w' and ${x}also $y and $z too} . qq{\n}
   . q{'$x' '$ x' '$  x' '${x}' '${ x}' '${  x}'}   . qq{\n}
   . q{'$y' '$ y' '$  y' '${y}' '${ y}' '${  y}'}   . qq{\n}
   . q{'$z' '$ z' '$  z' '${z}' '${ z}' '${  z}'}   . qq{\n}
   ;
 print qq{$interpol};
 ;;
 my $scalar = qr{ (?<! \\) \$ \s* (?: { \s* (\w+) } | (\w+) ) }xms;
 $interpol =~ s{ $scalar }
               { my $s = qq{\$$^N};
                 my $e = eval $s;
                 $@            ? qq{[['$s' non-existant]]} :
                 ! defined($e) ? qq{[['$s' undefined]]}    :
                                 $e;
               }xmseg;
 ;;
 print qq{'$interpol'};
"
hi \$w from '$w' and ${x}also $y and $z too
'$x' '$ x' '$  x' '${x}' '${ x}' '${  x}'
'$y' '$ y' '$  y' '${y}' '${ y}' '${  y}'
'$z' '$ z' '$  z' '${z}' '${ z}' '${  z}'

'hi \$w from 'YUU' and EKSalso [['$y' undefined]] and [['$z' non-exist
+ant]] too
'EKS' 'EKS' 'EKS' 'EKS' 'EKS' 'EKS'
'[['$y' undefined]]' '[['$y' undefined]]' '[['$y' undefined]]'
 '[['$y' undefined]]' '[['$y' undefined]]' '[['$y' undefined]]'
'[['$z' non-existant]]' '[['$z' non-existant]]'
 '[['$z' non-existant]]' '[['$z' non-existant]]'
  '[['$z' non-existant]]' '[['$z' non-existant]]'
'

[download]

In reply to Re: super-simple regex-based template eval --- handling undefined substitutions by AnomalousMonk
in thread super-simple regex-based template eval --- handling undefined substitutions by iaw4

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