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";