You may want to just open a pipe:

open CSH, "| csh -f -DHOME=new -DPATH=." || die "Open failed";
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It sounds like you have the whole picture:

my $child = fork;
die "Fork failed" if not defined $child;
if ( $child ) {
   # parent stuff
} else {
   #child
    %ENV = ( what => 'ever', you => 'need' );
   exec( ... ) || die "exec failed: $!";
}
[download]

Be well,
rir