LWP::RobotUA may not need it there. But for longer strings, it's just a good habit to be in. For example,

if ($string =~ m/@a/) {
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you'll get

Possible unintended interpolation of @a in string at ./y.pl line 7.
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as a warning message under use warnings; (which you are doing, right?). But with the \, you'll get exactly what you wanted - an @-sigil followed by the letter a.