Since most people tend to write the "if" and and conditional on the same line, the following one-liner should work for most Perl scripts:

perl -ne'if(m/if\s*\(.+?=[^~].+?\)/){print "Possible use of = instead 
+of == in line $.\n"}' script.pl
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Of course this won't catch cases where the if and conditional are not on the same line, or constructs such as:

something() if $var1 = $var2;
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Covering more cases will require a full blown script... or perhaps a new CPAN module :-)