or just extract the ascii that they contain

This basic code will print an ASCII printable character, or '?' if outside the range:

use warnings;
use strict;

while(<>) {

    for my $char (split '') {
        my $ord = ord($char); 
        print $ord > 31 && $ord < 128 ? $char : '?'
    }
}
[download]

Run it from the command line like this: myscript.pl filename
Note that this only covers ASCII, if you are using ISO Latin 1 (which you might be) then change the 128 to 256.

Update: The code above is not ideal because it relies on new-lines to terminate each "record", which might not exist. Might be better with:

use warnings;
use strict;

open (my $handle, '<', $ARGV[0]) or 
    die "Unable to open $ARGV[0]: $!";

binmode $handle;
   
while(read ($handle, my $buffer,80)) {

    for my $char (split '', $buffer) {
        my $ord = ord($char); 
        print $ord > 31 && $ord < 128 ? $char : '?'
    }
    print "\n";
}
[download]

Run it in the same way as before.