Well if you don't allow HTML, I would use ~= s/// to strip out the HTML, then just check the string length with length(), no regex needed for that.

# "Find a < or </ followed by anything 
# NOT a > and sub it with NOTHING." 
$string =~ # store the results in $string: 
s/ # substitute 
< # A beginning tag carat alone or . . . 
\/* # next to a closing slash if there is one (*) 
([^>])+ # Followed by one or more of anything NOT a >. 
> # Followed by end of tag. 
//igx; # sub with nothing, case-insensitive, global, allow whitspace in regex. 

# in one line:
$string =~ s/ < \/* ([^>])+ > //igx;

if ((length($string) < 100) or ($length($string) > 3000)) {
print "<p>String is too long or too short.</p>";
}