if is not an operator. It's part of the syntax of an if statement. An if statement evaluates its condition first, then evaluates the rest if and only if the condition evaluated to something true. 

EXPR if CONDEXPR;

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is exactly the same as 

(CONDEXPR) and EXPR;

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By the way, 

my $var ... if ...;

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is not allowed. (Same goes for unless.) You should be using 

my $message;
$message = "Congrats" if defined $reward;

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or 

my $message = $reward ? "Congrats" : undef;

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In reply to Re: order of operations with conditionals at the end by ikegami
in thread order of operations with conditionals at the end by BeneSphinx

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