I don't see the point of the prototype. It actually makes your example caller bigger!

my @init_to_toto = qw{ a b c f t u };

my %hash = ( y => 'pre', z => 'pre', );
mass_init(\%hash, \@init_to_toto,  'toto');
mass_init(\%hash, [qw( b g k p )], 'TITI');

use Data::Dumper;
print Dumper \%hash;

sub mass_init {
  my ($hashref, $arrayref, $string) = @_;
  # $hashref->{$_} = $string for @$arrayref;
  @{ %$hashref }{ @$arrayref } = ($string) x @$arrayref;
  }
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