I was surprised to see that it generates a warning without strict or warnings

That's one of default warnings, known as mandatory warnings before use warnings; was introduced. These warnings will be enabled by default, but can be controlled with -X switch or warnings. So the following code will issue a warning as you mention:

perl -e 'my $x; if ($x = 1) {}'
[download]

But this code won't:

perl -Xe 'my $x; if ($x = 1) {}'
[download]

Neither these snippets:

# disable all warnings
no warnings;
my $x;
if ($x = 1) {}

# disable only warnings regarding syntax
no warnings 'syntax';
my $x;
if ($x = 1) {}
[download]

References: perllexwarn, perldiag

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