Yep, your example shows what I'm talking about :-) Remember Perl's special false value will look like both the empty string and the number zero at the same time (it's basically a dualvar), in your YAML it's showing up as its string version. To identify Perl's special false value, you actually have to look a tiny bit into the internals. The first example below shows that in list context, a failed match is returning the empty list (which in a hash translates to undef; add -w to your first example and you get Odd number of elements in anonymous hash). The second example shows that forcing scalar context on the match causes it to return Perl's special false value.

$ perl -MDevel::Peek -le 'my @x = /abc/; Dump(@x)'
SV = PVAV(0x51f85b346e78) at 0x51f85b3c66b8
  REFCNT = 1
  FLAGS = ()
  ARRAY = 0x0
  FILL = -1
  MAX = -1
  FLAGS = (REAL)

$ perl -MDevel::Peek -le 'my @x = scalar(/abc/); Dump(@x)'
SV = PVAV(0x579bb31a2e38) at 0x579bb31c86a8
  REFCNT = 1
  FLAGS = ()
  ARRAY = 0x563bc3d01720
  FILL = 0
  MAX = 3
  FLAGS = (REAL)
  Elt No. 0
  SV = PVNV(0x579bb31a1f40) at 0x579bb31a15a8
    REFCNT = 1
    FLAGS = (IOK,NOK,POK,pIOK,pNOK,pPOK)
    IV = 0
    NV = 0
    PV = 0x579bb31cd6e4 ""\0
    CUR = 0
    LEN = 10
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That’s very interesting…

perl -MYAML -E '$vol = "asdf"; $x = { moo => scalar($vol =~ m/^HOLDING
+/) }; say $x->{moo} == 0'
1
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But also, to me at least, very surprising and I wouldn’t want to see it in code.

I guess it depends: mapping it to a different value, as in bool ? 1 : 0, will definitely make it more clear (and also force scalar context). But using scalar is better than not using it at all, since if you've got a hash { moo => $vol =~ m/^HOLDING/, foo => 'bar' } that's asking for trouble (and in some cases, security holes, like with CGI.pm's param).

Update: Corrected the example hash