Check the documentation for the alarm function in perldoc.

Here is some example code for your situation;

my $timeout = 5;
eval {
     local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
     alarm $timeout;
     @data = $tnet->cmd('show ip cache flow');
     alarm 0;
};
if ($@) {
     die unless $@ eq "alarm\n";   # propagate unexpected errors
     # timed out
}
else {
     # didn't
}
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