First some overview:
self->{xslfile} is the xslt template
self->{template} is the data that will be parsed into the template.

Both parameters are filled in before running this snippet, and the glitch was only occassional. Here we go:

    print STDERR "FIRST $self->{template}\n";
    my $xslt = XML::XSLT->new(Source =>  $self->{xslfile});
    print STDERR "SECND $self->{template}\n";
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Later during the $xslt->serve(Source => $self->{template}) expat would intermittently complain about malformed XML document, always on the 56th byte, no matter what I did to the content before trying to output. Always worked fine when I ran the same content as a cgi.

The fix? instead of     my $xslt = XML::XSLT->new(Source =>  $self->{xslfile}); I changed to     my $xslt = XML::XSLT->new(Source =>  ($self->{xslfile})); That is, I wrapped $self->{xslfile} in parens.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=freebsd, osvers=4.0-current, archname=i386-freebsd
    uname='FreeBSD freefall.FreeBSD.org 4.0-current FreeBSD 4.0-curren
+t #0: $Date$'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='undef', gccversion=2.95.2 19991024 (release)
    cppflags=''
    ccflags =''
    stdchar='char', d_stdstdio=undef, usevfork=true
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags ='-Wl,-E -lperl -lm '
    libpth=/usr/lib
    libs=-lm -lc -lcrypt
    libc=, so=so, useshrplib=true, libperl=libperl.so.3
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' -Wl,-R/
+usr/lib'
    cccdlflags='-DPIC -fpic', lddlflags='-Wl,-E -shared -lperl -lm '


Characteristics of this binary (from libperl): 
  Built under freebsd
  Compiled at May 25 2004 21:10:23
  @INC:
    /usr/local/lib/perl5/site_perl/5.005/i386-freebsd
    /usr/local/lib/perl5/site_perl/5.005
    .
    /usr/libdata/perl/5.00503/mach
    /usr/libdata/perl/5.00503
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