# Edit-in-place in multiple files - one file at the time

{
    local $^I = q{};
    local @ARGV = @files;
    while(defined(my $line = <>)){
        # do something with $line
    }
    continue {
        print $line;
    }
}

##</code><code>##

# Read multiple lines from multiple files at the same time

my @files = @ARGV;

my @fh;
my $i = 0;
foreach my $file (@files) {
    -f -r $file or next;
    open $fh[$i++], '<', $file
      or die "Cannot open $file: $!";
}

while (1) {
    my @lines;
    foreach my $i (0 .. $#fh) {
        ref $fh[$i] eq 'GLOB' or next;
        push @lines, scalar readline $fh[$i];
        if (eof $fh[$i]) {
            close $fh[$i];
            $fh[$i] = undef;
        }
    }
    @lines or last;
    foreach my $line (@lines) {
        # do something with $line
    }
}