\t and \n are only interpreted as tabs and newlines in double-quoted text:

$nl = "\n";   # newline
$nl = '\n';   # backslash n
$nl = qq(\n); # newline
$nl = q(\n);  # backslash n
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Basically, this means that your literal text is either quoted wrong (if it's a literal string in your code) or it has a literal \n (not a newline) in the text (if you're reading it from a file / somewhere else).

see also Quote and Quote-like Operators in the perlop manpage.

update: If you *really* want to interpret all \n as newline and \t as tabs from your input, you can do something like this:

for (@input) {
  s/\\n/\n/g; # replace \n with newline
  s/\\t/\t/g; # replace \t with tab
}
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Note: this doesn't take quoting into account (\\n will be \ followed by a newline)