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while (my $line = readline($FH[$i])) {
   ...
}

my $fh = $FH[$i];
while (my $line = <$fh>) {
   ...
}

open(FILE_A,">","a.txt") || ...;
open(FILE_B,">","b.txt") || ...;
my @FH=(*FILE_A,*FILE_B);

open(my $FILE_A,">","a.txt") || ...;
open(my $FILE_B,">","b.txt") || ...;
my @FH=($FILE_A,$FILE_B);