I'd rather go for extra semicolon:

• %set= map {; $_ => foo($_) } @list;

demo with perl -de0

  DB<45> x map {a=>1} 1..3;                      # seeing hash, but co
+mma missing
syntax error at (eval 55)[c:/Strawberry/perl/lib/perl5db.pl:738] line 
+2, near "} 1"
  DB<46> x map {a=>1}, 1..3;                     # seeing hash again
0  HASH(0x2f501e8)
   'a' => 1
1  HASH(0x2f4a1e8)
   'a' => 1
2  HASH(0x2f4d0e0)
   'a' => 1
  DB<47> x map {;a=>1} 1..3;                      # seeing code
0  'a'
1  1
2  'a'
3  1
4  'a'
5  1
  DB<48>
[download]

YMMV! :)

edit