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#!/usr/bin/perl

...
# system "/bin/echo hello";
# or 
# my $out = `/bin/echo hello`;

/* fakesys.c */
#include <unistd.h>   /* for execvp */
...
    /* execute original lib function */
    return real_execvp(file, argv);
}

$ gcc -D_GNU_SOURCE -fPIC -shared fakesys.c -o libfakesys.so

$ LD_PRELOAD=./libfakesys.so perl ./685699.pl

calling execvp("/bin/echo", ...)
hello