You're asking to do something that's the very definition of looping without using a loop. What do you actually want?

I also sometimes wonder why we regularly get questions how to do things "without a loop"...  but I think what people are typically looking for is just some kind of syntactic shortcut that avoids having to spell out things the cumbersome way. Like, for example in this case:

my @a = qw(foo bar baz);
my @b = (1, 42, 99);

# unwanted (considered too clumsy):

for my $i (0..$#a) {
    $h{$a[$i]} = $b[$i];
}

# or
$h{$a[0]} = 1;
$h{$a[1]} = 42;
$h{$a[2]} = 99;

# or
($h{$a[0]}, $h{$a[1]}, $h{$a[2]}) =  (1, 42, 99);


# wanted:

@h{@a} = @b;

@h{@a} = (1, 42, 99);
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They don't particularly care about the internal implementation, i.e. that something behind the scenes of course will have to do the looping... At least, that's my guess.

It's often about perceived performance, I believe.

Sure, it's a clarity question here. What does that have to do with avoiding loops? I'd take

$b{$_}=1 for @a;
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over

@b{@a} = (1)x@a;
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even though the other might be considered to have no loop. (It has three while the first has two.)

Actually, I'd take the following since it incorporates the my:

my %b = map { $_ => 1 } @a;
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Impossible. You're asking to do something that's the very definition of looping without using a loop. What do you actually want?

At university I learned that loops can be replaced by recursivity. So the simple code below fulfills the requirements of the OP (one line, no loop) ... but maybe isn't what he wants. ;-)

However this statement stems from a course of theoretical computer science. Don't use the code below in a productive environment. It is slower and takes much more resources than the other, straightforward solutions presented in this thread!

You have been warned! Rata

use strict;
use warnings;

my @x = qw ( a b c d e ff ggg hhhh iiii );
my %y;

# Tata!!! Here comes the requested line of code:

noLoop(); sub noLoop { $y{pop(@x)} = 1; noLoop() if (scalar(@x)); }  #
+ btw.: using global vars in subs is usually bad style ;-)

print keys(%y), "\n";
exit 0;
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Yes, there's a lot of different ways of writing loops.

Note that your code can be simplified to

my %y; $y{shift(@x)} = 1 while @x;
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or the non-destructive

my %y = map { $_ => 1 } @x;
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