use warnings FATAL => qw{ uninitialized };

my $abc = "ABC";
my $def = "DEF";
## no $ghi is defined.

my $funnystring= '
   hello $abc and $def and $ghi';
$funnystring =~ s/(\$\w+)/"defined($1) ? $1 : 'unknown $1'"/gee;
print $funnystring;