If you use strict and warnings (or at least warnings), you should get a warning like:

Found = in conditional, should be == at scope_test.pl line 5.
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For example, in the following code:

use strict;
use warnings;

while (my $y = 1) {
    $y = 0;
    print "hello (y = $y)\n";
}
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You will get the warning about "= in conditional".

It's also interesting to note that this test program, which is essentially equivalent to:

use strict;
use warnings;

my $y = 0;
while ($y = 1) {
    $y = 0;
    print "hello (y = $y)\n";
}
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will never terminate, even though $y is set to zero each time, because it's set to 1 immediately before the condition of the while statement is evaluated.

I find it interesting that so many people jumped on the "you don't want to do an assignment in a conditional" bandwagon without even knowing what the right hand side of the assignment is. There are times when an assignment in a conditional is exactly what you want. Here is an example that I use almost daily with DBIx::Class...

my $rs = $schema->resultset( 'Foo' );
while ( my $record = $rs->next ) {
    # do something with the record
}
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The warning you mention only occurs if the right hand side of the assignment is a constant, and the OP didn't include what was in the RHS.

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We're not surrounded, we're in a target-rich environment!

I find it a bit strange that you assume that someone who askes about the meaning of a warning does not "use warnings".

I also find it strange that a node that completely ignores the question of the op gets such a high reputation.

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_Search, Ask, ^{Know something completly different}

will never terminate, even though $y is set to zero each time, because it's set to 1 immediately before the condition of the while statement is evaluated.

Yep, one would want to use redo there, in that case.