use 5.020;
use warnings;

my %h = ( foo => 1, bar => 2 );

$_ = 'foobar';
s/(foo)/$h{'$1'}/;
say $_;

$_ = 'foobar';
s/(foo)/$h{$1}/;
say $_;

my $foo = 'foo';
say "$h{'$foo'}bar";

##</code><code>##

Use of uninitialized value $h{"$1"} in substitution iterator at extr.pl line 7.
bar
1bar
Use of uninitialized value $h{"$foo"} in concatenation (.) or string at extr.pl line 15.
bar