or

my %hash = map { my $foo = 'longstring'.$_.'more';
                 $_ => { key1 => $foo,
                         key2 => alter($foo) }     }
           @list;
[download]

or (the wacky)

my %hash = map { $_->[0] => { key1 => $_->[1],
                              key2 => alter($_->[1]) } }
           map { [ $_ => 'longstring'.$_.'more' ] }
           @list;
[download]

or (the efficient)

my %hash;
foreach (@list) {
   my $foo = 'longstring'.$_.'more';
   $hash{$_} = {
      key1 => $foo,
      key2 => alter($foo),
   };
}
[download]

or (using auto-vivification)

my %hash;
foreach (@list) {
   my $foo = 'longstring'.$_.'more';
   $hash{$_}{key1} = $foo;
   $hash{$_}{key2} = alter($foo);
}
[download]

or

my %hash;
foreach my $key (@list) {
   foreach ('longstring'.$key.'more') {
      $hash{$key} = {
         key1 => $_,
         key2 => alter($_),
      };
   }
}
[download]

or

my %hash;
foreach my $key (@list) {
   foreach ('longstring'.$key.'more') {
      $hash{$key}{key1} = $_;
      $hash{$key}{key2} = alter($_);
   }
}
[download]

or (in two steps)

my %hash = map { $_ => { key1 => 'longstring'.$_.'more' } }
           @list;

$hash{$_}{key2} = alter($hash{$_}{key1})
   foreach keys %hash;
[download]

Updated: Added more alternatives.

Updated: Fixed bug. Changed $_ to $key in the concatenation in later snippets.

Updated: Added more alternatives.