iam getting error at @INFFiles = `dir /b $filespec`;

Use opendir, readdir and closedir or glob instead.

iam not understanding this part $VersionNumber =~ s/[^\d+\.\d+\.\d+\.\d+]//g;

[^\d+\.\d+\.\d+\.\d+] is a character class that matches any single character that is not the character '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+' or '.' which could more simply be written as [^\d+.].    The substitution operator replaces each character that matches with nothing globally with the result that $VersionNumber will only contain the twelve characters '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+' and '.'.