As toolic points out, the braces are usually employed when accessing a variable and are necessary when you have to interpolate a variable into a string and the variable name has to be disambiguated from following text. An alternative is to use concatenation. In the following code the braces are needed because there is no $preffish variable.

knoppix@Microknoppix:~$ perl -E '
> @prefs = qw{ cat dog monk };
> foreach $pref ( @prefs )
> {
>     say qq{${pref}fish};
>     say $pref . q{fish};
> }'
catfish
catfish
dogfish
dogfish
monkfish
monkfish
knoppix@Microknoppix:~$
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I hope this is helpful.