THIS IS NOT A SHELL SCRIPT! DON'T ENCLOSE VARIABLES IN DOUBLEQUOTES!

Sorry for screaming, but foo( "$varname") or $hash{"$varname"} is a sure sign you misunderstood something. And you are begging for problems. If the variable contains a number, then by enclosing it in double quotes you force perl to convert it to a string and make a copy of that string. If it's already a string you only make a copy. Just a waste, BUT if it's a reference you kill the reference. The thing you end up with will no longer be a reference. It will be just a string that will look a bit as if it was a reference but it will not work:

my %h = (a => 5, b => 9);
my $ref = \%h;
print "\$ref->{a}=$ref->{a}\n";

my $not_ref = "$ref";
print "\$not_ref->{a}=$not_ref->{a}\n";

print "Even though $ref == $not_ref\n";
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While there are valid reasons to force stringification to a variable, they are very very rare! So please drop all your doublequotes around your variables!

Update: Anno is right, $ref == $not_ref evaluates to false, $ref eq $not_ref would evaluate to true. I meant

print "Even though $ref looks like $not_ref\n";
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I should have written it like that.