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# WARNING: THIS CODE HAS A SECURITY HOLE
print "Which file to show? ";
...
die "bad filename" if $f=~/\.\./; # don't allow updirs
system("cat /sharedfiles/$f")==0
    or die "command failed, \$?=$?";

system("cat", "/sharedfiles/$f")==0
    or die "command failed, \$?=$?";
# OR
system({"cat"} "cat", "/sharedfiles/$f")==0
    or die "command failed, \$?=$?";

use IPC::Run3 0.047 'run3';
my $cmd = ['cat','-nE'];
...
die "run3 failed, \$?=$?" unless $?==0;
print "# STDOUT:\n$stdout";
print "# STDERR:\n$stderr";

use Capture::Tiny 'capture';
my ($stdout, $stderr, $exit) = capture {
...
die "system failed, \$?=$exit" unless $exit==0;
print "# STDOUT:\n$stdout";
print "# STDERR:\n$stderr";

use IPC::System::Simple qw/systemx capturex/;
print "# systemx:\n";
systemx 'echo', 'Hello,', 'World!';
my $stdout = capturex 'echo', 'Hello,', 'World!';
print "# capturex: $stdout";

$ cat lengthsum.pl 
#!/usr/bin/env perl
...

$ cat /usr/share/dict/words | ./lengthsum.pl 
839677

use 5.008;
my @cmd = ('cat', '/usr/share/dict/words');
...
}
close $fh or die $! ? $! : $?;
print "$sum\n";

use IPC::Run qw/ run new_chunker /;

...
        $sum+=length $line;
    } or die $?;
print "$sum\n";