my $content = <<CONTENT;                                             
foo somestuff
zimzam someotherstuff
bar evenmorestuff

foo
bar

zimzam

foo zimzam bar
CONTENT

my $orig_zimzam = 'zimzam';
my $rep_zimzam  = 'zoomzam';
$content =~ s/(foo(?!bar).*?)\Q$orig_zimzam\E(.*?bar)/$1$rep_zimzam$2/sg;
print $content;

##</code><code>##

foo somestuff
zoomzam someotherstuff
bar evenmorestuff

foo
bar

zoomzam

foo zimzam bar

##</code><code>##

foo somestuff
zoomzam someotherstuff
bar evenmorestuff

foo
bar

zimzam

foo zoomzam bar