while(EXPR) has special magic for <> aka readline, but only if it's the sole EXPR. °

while (<>) {

is a shorthand for

while ( defined( $_ = <> ) ) {

Personally I'd write

while (<>) {
    last if /pattern/;
    ...
}
[download]

I find this more readable. YMMV!

HTH :)

update

B::Deparse can help you explore the difference

C:\>perl -MO=Deparse -e"while (<>) {}"
while (defined($_ = readline ARGV)) {
    ();
}
-e syntax OK

C:\>perl -MO=Deparse -e"while (<> and !/pattern/) {}"
while (readline ARGV and not /pattern/) {
    ();
}
-e syntax OK

C:\>
[download]

update

°)

Like the <EXPR> operator, if a readline expression is used as the condition of a while or for loop, then it will be implicitly assigned to $_. If either a readline expression or an explicit assignment of a readline expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.

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