Maybe you should write some code that fails as you describe? Adding strictures (use strict; use warnings;) to your code and running it I get:

use strict;
use warnings;

my %something;
my $key = "that's it!"; #contents from file foo.txt
my $value = "foo"; #filename
$something{$key} = $value;
print $something{$key};
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which prints:

foo
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just as I would expect. Can you show me equivelent code that fails?