while (my ($test1, $test2) = (scalar <$fh1>, scalar <$fh2>)) {
   last unless defined $test1 or defined $test2;
   ...
}

##</code><code>##

while ( my $test1, $test2 ) = (scalar <$fh1> // (), scalar <$fh2> // () ) {
    ...
}

##</code><code>##

sub read_double_file {
    my ($fh1, $fh2)                 = @_;
    my ($line_file_1, $line_file_2) = (scalar <$fh1>, scalar <$fh2>);

   (defined $line_file_1 or defined $line_file_2) ? ($line_file_1 //= '', $line_file_2 //= '') : ();
}

while ( my ($line_from_1, $line_from_2) = read_double_file($fh1, $fh2) ) {
    ...
}