It is not clear exactly what you are trying to achieve. It looks like you want to create an array @ab that contains just the keys from the hash %abc, each with a hypen prepended.

use strict;
use warnings;

my %abc = (
            to => 'def',
            from => 'ghi',
            );
my @ab;
for ( keys %abc )
{
    push @ab, "-$_";
}

print "$_\n" for @ab;


#Output
-to
-from
[download]

Will do that, or you can use map to do the same thing.

use strict;
use warnings;

my %abc = (
            to => 'def',
            from => 'ghi',
            );
my @ab = map { "-$_" } keys %abc;

print "$_\n" for @ab;
[download]

As doing this disconnects the modified keys from their previously associated values, I guess this is not really what you want. I think it is more likely that you want to just modify the keys in the hash, or more simply create a new hash with the modified keys.

my %ab = map { '-' .$_ => $abc{$_} }  keys %abc;
[download]