Well, if there's a source filter, or if 'some expression' invokes some XS code that walks the parse tree, there might be a difference. But normally, there won't be any semantic difference. There will be a small speed difference though, since the second expression will store an undefined value in $x before assigning 'some expression':

#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;

use Benchmark qw /cmpthese/;

cmpthese -1 => {
    single  => 'my $x = "foo"',
    double  => 'my $x; $x = "foo"',
};

__END__
            Rate double single
double 2710223/s     --   -14%
single 3143931/s    16%     --
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