In general you need to read the whole string one character at a time and if you come across something that doesn't make sense in your character encoding, it's binary. Otherwise it's text. In the case of ASCII text, the characters that don't make sense are most of the control characters and 0x7F - 0xFF. The following control characters are usually considered OK in text data:
- 0x09 - tab
- 0x0A - line feed
- 0x0C - form feed
- 0x0D - carriage return
A regex-ish way of detecting non-ASCII data based on that might be:
print ($text =~ /[^\x09\x0a\x0c\x0d\x20-\x7e]/) ?