This won't compile:

my $what = "fred|barney";
...
if ($_ =~ ($what){3}) { ... }
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Dropping the () around $what will make it compile, but that really changes what the code is saying.

if ($_ =~ $what{3}) { ... }
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is going to try to find the value in %what that is assosciated with the key '3' and use that as a regex. Unless you have some %what hash, this will be undef and will be intrepreted as

if ($_ =~ //) { ... }
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which will match anything, including nothing. Essentially, that's just

if (1) { ... }
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and you most likely do not want that.

This

my $what = "(fred|barney){3}";
...
if ($_ =~ $what) { ... }
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probably does the closest to what you want (match three occurrences of 'fred' or 'barney' in any order. This works because =~ treats whatever it finds on the right hand side as a RE (regular expression).

You could get the same behavior from either of your examples by using the m// operator (the 'm' can be omited if you are using slashes as the delimiter).

if ($_ =~ /($what){3}/) { ... }
....
if ($_ =~ /$what/) { ... }
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If you do this, then you can eliminate the '$_ =~' as it is redundant:

if (/($what){3}/) { ... }
...
if (/$what/) { ... }
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