The ; 1 is to make sure I get a true value no matter what the function returns. If it fails, I think there are cases where the block still might return a true value, but I don't know.

But as it, it's an example of cargo-cult. Sure, $res is true if and only if the eval doesn't fail, but you're not using its result. Instead, you rely on $@ to determine failure. But the entire '; 1' idiom is there because relying on $@ to determine failure can trigger both false positives and false negatives. The correct idiom is:

my $res = eval {...statements...; 1};
unless ($res) {
    ... eval failed ...
}
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relying on $@ to determine failure can trigger both false positives and false negatives

Can you explain how?

The usual way to handle errors from functions that die to signal them is:

eval { do_whatever() };
if ($@) {
  # handle error
  ...
}
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or if you want to use the value returned by do_whatever():

my $res = eval { do_whatever() };
if ($@) {
  # handle error
  ...
}

# use $res here;
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I believe that the 1 usage comes from...

eval { do_whatever(); 1 } or print "failed!\n";
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