writing $_ == 1 is dangerous because it is too easy to drop an = and assign rather than compare.

Some like to put constants on the LHS (1 == $_) to avoid that problem.

>perl -e"$_ = 0; die qq{uh oh!\n} if $_ = 1;"
uh oh!

>perl -e"$_ = 0; die qq{uh oh!\n} if 1 = $_;"
Can't modify constant item in scalar assignment at -e line 1, near "$_
+;"
Execution of -e aborted due to compilation errors.
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