#!/usr/bin/perl
system("perl script1.pl input.txt output.txt"); # A long process
system("perl script2.pl output.txt finalout.txt");

##</code><code>##

script2.pl : failed to open input file output.txt : No such file or directory

##</code><code>##

#!/usr/bin/perl
system("perl script1.pl input.txt output.txt");

WL:
while(0) {
   sleep 10;
   if ( -e "output.txt") {
    system("perl script2.pl output.txt finalout.txt");
    last WL;
   }
}