The first mistake is open (DAT$i, "T$iT.txt");.

You can't just use variables as part of other variables. And "T$iT.txt" searches for the variable $iT, and can't find it.

You can do this instead:

# always start your scripts with these two lines...
use strict;
use warnings;

# and declare your variables

foreach my $i (2 .. 4) {
    my $filename = 'T' . $i . 'T.txt';
    open (my $file, $filename) or die "Can't open '$filename': $!";
    my @a = <$file>;
    print @a;
    close $file;
}
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