In the Perl script "test.pl":
        ...		
        my $WEBROOT = <WEBROOT_PATH>;
        ...

To force variable substitution try:
        $export WEBROOT_PATH=/usr/local/webroot
        $sed 's/<WEBROOT_PATH>/$WEBROOT_PATH/' < test.pl > out

But this just gives:
        my $WEBROOT = $WEBROOT_PATH;

However:
        $export WEBROOT_PATH=/usr/local/webroot
        $eval sed 's%\<WEBROOT_PATH\>%$WEBROOT_PATH%' < test.pl > out
#        ====

That works fine, and gives the expected substitution:
        my $WEBROOT = /usr/local/webroot;


### Correction applied to original example by Paulo Marcel Coelho Aragao.