The script provided already does exactly that. It can be proven by changing
open(EXECOPU, '| execopu -text')
to
open(EXECOPU, '| cat')

$ cat 449609.pl
#!/usr/bin/perl

my @sitenumbers = (7,12);

#open(EXECOPU, '| execopu -text')
open(EXECOPU, '| cat')
or die("Can't run execopu: $!\n");

print EXECOPU "cell\n";

foreach my $site_number (@sitenumbers){

print EXECOPU <<"EOF";
u
csno = $site_number
ccuoosmin_3g1x = ""
ccuoos_3g1x = ""
u
EOF

}
$ perl 449609.pl
cell
u
csno = 7
ccuoosmin_3g1x = ""
ccuoos_3g1x = ""
u
u
csno = 12
ccuoosmin_3g1x = ""
ccuoos_3g1x = ""
u
[download]