Each example must be run separately. You're getting the value of $x defined from the previous test. If you run the 2nd one by itself, however:

$ perl -le '
    BEGIN { my $x = 1; }
    print defined $x
          ? "defined"
          : "undefined";'
undefined
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And with strict:

$ perl -Mstrict -le '
    BEGIN { my $x = 1; }
    print $x;'
Global symbol "$x" requires explicit package name at -e line 2.
Execution of -e aborted due to compilation errors.
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