If user tries to enter 11th digit a pop up error message should be generated refraining the user to do so. Can I do this in Perl?
Yes, if you write a browser in perl ... or embed perl in a browser (like firefox) ... or use client-side javascript (or vbscript...). Here is sample code.

 
print "\n\n<br><br><b> Phone Number (Please indicate whether this is a
+ business, home or cell number): &nbsp&nbsp </b>";    
print "<input type=text size=25 maxlength=23 name = \"phone\" value = 
+\"$phone\" SOMETHINGSOMETHINGJAVASCRIPTGOESHERE>";
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