map says that both EXPR and BLOCK are evaluated in list context. Using a parenthetical list is ambiguous as they are taken as enclosing the arguments to map I think.

$ perl -le '
> %h = map ( $_, 2 * $_ ), 1 .. 3;
> print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;'
0 - 
$
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Using an anonymous array emits a single scalar containing the reference so you then need another map to dereference it.

$ perl -le '
%h = map @$_, map [ $_, 2 * $_ ], 1 .. 3;
print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;'
1 - 2
2 - 4
3 - 6
$
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You could construct a subroutine that returns a list in the fly.

$ perl -le '
%h = map sub { return ( $_, 2 * $_ ) }->( $_ ), 1 .. 3;
print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;'
1 - 2
2 - 4
3 - 6
$
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Simplest is to use map BLOCK list as already suggested.

$ perl -le '
%h = map { $_, 2 * $_ } 1 .. 3;
print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;'
1 - 2
2 - 4
3 - 6
$
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I hope this is of interest.

Cheers,

JohnGG

Update: Couldn't see the wood for the parentheses, just add an extra pair so there's no ambiguity.

$ perl -le '
%h = map ( ( $_, 2 * $_ ), 1 .. 3 );
print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;'
1 - 2
2 - 4
3 - 6
$
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Update 2: Another way is to use a do block.

$ perl -le '
> %h = map do { ( $_, 2 * $_ ) }, 1 .. 3;
> print qq{$_ - $h{$_}} for sort {$a <=> $b} keys %h;'
1 - 2
2 - 4
3 - 6
$
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