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

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

sub read_double_file {
    my ($fh1, $fh2)                 = @_;
...
while ( my ($line_from_1, $line_from_2) = read_double_file($fh1, $fh2)
+ ) {
    ...
}