This depends on just how you define 'everything except sequence xyz'.

One way (note that a piece of 'xyz', the 'yz', is captured – after all, 'yz' is not 'xyz'!):

>perl -wMstrict -le
"my $s = 'abcxyzdef';
 my $exclude = qr{ xyz }xms;
 my $all_not_xyz = qr{ (?! $exclude) . }xms;
 my @matches = $s =~ m{ ($all_not_xyz+) }xmsg;
 print qq{@matches};
"
abc yzdef
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Another way (note that the 'xyz' sequence is entirely excluded):

>perl -wMstrict -le
"my $s = 'abcxyzdef';
 my $exclude = qr{ xyz }xms;
 my @matches = split m{ $exclude }xms, $s;
 print qq{@matches};
"
abc def
[download]

These approaches generalize nicely to multiple exclusion sequences:

>perl -wMstrict -le
"my $s = 'abcFOOdefBARghi';
 my $exclude = qr{ FOO | BAR }xms;
 my $all_but = qr{ (?! $exclude) . }xms;
 my @matches = $s =~ m{ ($all_but+) }xmsg;
 print qq{@matches};
 @matches = split m{ $exclude }xms, $s;
 print qq{@matches};
"
abc OOdef ARghi
abc def ghi
[download]