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use Modern::Perl;
use warnings FATAL => qw{ uninitialized };
...
my $funnystring = 'hello $abc and $def and $ghi and $jkl';
$funnystring =~ s/(\$\w+)/defined eval $1 ? eval $1 : "unknown $1" /ge
+;
print $funnystring;

hello ABC and DEF and unknown $ghi and 0