foreach (@trt_files) # Each time round the loop, $_
                     # is set to an alias to one of the
                     # elements of @trt_files.
{
    $record = $_;    # $record now contains the _value_
                     # of $_. It is _not_ an alias into
                     # @trt_files.
    $record =~ s/date/$date/; # This changes $record
                              # (it doesn't change $_
                              # or @trt_files).
    print "$_\n";    # Prints the (unchanged) value of $_
}

##</code><code>##

foreach (@trt_files)
{
    s/date/$date/;
    print "$_\n";
}