foreach my $file (<*.txt>) {
   open(FILE,"< $file") or die "Cannot open $file - $!\n";
   # Read in from FILE and do stuff with it...
}

##</code><code>##

@ARGV = <*.txt>;
while (<>) {
    # Files will now be opened and read automagically, and
    # each line placed in $_.  You can see which file
    # you're currently reading from by looking at $ARGV.
}