autodie, IPC::System::Simple, eval

$ echo Hello >t1.txt

$ echo World >t2.txt

$ perl -wle " $ret = system q[diff -u t1.txt t2.txt >NUL 2>&1] ; print
+ $ret; print $?"
256
256

$ perl -Mautodie=system -wle " $ret = system q[diff -u t1.txt t2.txt >
+NUL 2>&1] ; print $ret; print $?"
"diff -u t1.txt t2.txt >NUL 2>&1" unexpectedly returned exit value 1 a
+t (eval 10) line 13
 at -e line 1

$ perl -MIPC::System::Simple=system -wle " $ret = system q[diff -u t1.
+txt t2.txt >NUL 2>&1] ; print $ret; print $?"
"diff -u t1.txt t2.txt >NUL 2>&1" unexpectedly returned exit value 1 a
+t -e line 1
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After consulting

$ diff --help |grep -i exit
Exit status is 0 if inputs are the same, 1 if different, 2 if trouble.
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and some docs, you don't need eval, you could write

$ perl -MIPC::System::Simple=system -wle " $ret = system [0,1], q[diff
+ -u t1.txt t2.txt >NUL 2>&1] ; print $ret; print $?"
1
256

$ perl -Mautodie=system -wle " $ret = system [0,1], q[diff -u t1.txt t
+2.txt >NUL 2>&1] ; print $ret; print $?"
1
256
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[0, 1] are acceptable exit value, any others and IPC::System::Simple will croak