There are a host of potential problems, security and otherwise, you may have to deal with for this, but let's ignore all those and attack the specific issue you've asked about.

You need Perl to perform variable interpolation on a previously existing string. You can accomplish this using a string eval, after formatting your input like a string to be interpolated. This means escaping potentially problematic characters first like backslashes and previously existing quotes.

my %TEST_HASH = (TEST_KEY => 'TEST_VALUE');
my $cmd = '/bin/touch $TEST_HASH{"TEST_KEY"}';

$cmd =~ s/\\/\\\\/g;
$cmd =~ s/"/\\"/g;
$cmd = eval qq{"$cmd"} or die $@;

print $cmd
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Please don't run your intended code on any machine you care about security on, because this is pretty much the definition of injection and privilege escalation.