You should

use strict;
use warnings;
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(and diagnostics if the error messages aren't clear enough).

Here's one problem that would've been revealed by strict/warnings

C:\>more client_reg.pl
sub hi{print q,hi,}

C:\>perl -e" do (client_reg.pl);hi()"
Undefined subroutine &main::hi called at -e line 1.

C:\>perl -we" do (client_reg.pl);hi()"
Unquoted string "pl" may clash with future reserved word at -e line 1.
Undefined subroutine &main::hi called at -e line 1.

C:\>perl -Mstrict -we" do (client_reg.pl);hi()"
Bareword "client_reg" not allowed while "strict subs" in use at -e lin
+e 1.
Bareword "pl" not allowed while "strict subs" in use at -e line 1.
Execution of -e aborted due to compilation errors.

C:\>perl -Mstrict -we" do q(client_reg.pl);hi()"
hi
C:\>
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do can also fail to read and execute a file, so you should check for that (you can read more about do in the docs: 'perldoc -f do').