Are you trying to send text to the execopu program? If so, check out Perl's open(FH, '|...') function:

#!/usr/bin/perl

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

my @site_numbers = (7, 12);

print EXECOPU "cell\n";

foreach my $site_number (@site_numbers) {
   print EXECOPU <<"EOF";
u
csno = $site_number
ccuoosmin_3g1x = ""
ccuoos_3g1x = ""
u
EOF
}
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You might also want to check our IPC::Open2, IPC::Open3 and IPC::Run, especially if you want to capture the output.