$ perl -e '$a="abc"x500;open A,"<",\$a;$x=-T(A); print +(defined($x)?$x?"text":"bin":"undef"),$/;close A'
undef
$ perl -e '$a="abc";open A,"<","/tmp/index.html";$x=-T(A); print +(defined($x)?$x?"text":"bin":"undef"),$/;close A'
text