My intuition would expect:

my $s = 'ABCDE';
print substr($s,0,2)='',$/;    # EXPECT '', GET 'cd'?
print substr($s,0,2)='foo',$/; # EXPECT 'foo', GET 'fo'?
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..in parallel with all other assignments:

my $s = 'ABCDE';
print $s='',$/;                # EXPECT ''
print $s='foo',$/;             # EXPECT 'foo'
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perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^`+*^e v^#$&V"+@( NO CARRIER'