use Modern::Perl;
use warnings FATAL => qw{ uninitialized };

my $abc = "ABC";
my $def = "DEF";
## no $ghi is defined.
my $jkl = 0;

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