If you want to replace a substring bounded by other substrings:

my $alpha = join '', ('a'..'z');  # $alpha = 'abcdefghijklmnopqrstuvwx
+yz'
$alpha =~ s/(?<=def).*(?=uvw)//;  # remove everything preceeded by the
+ substring 'def'
                                  #   and succeeded by the substring '
+uvw'
print $alpha;                     # $alpha = 'abcdefuvwxyz'
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Consult perlre and perlretut for more on the formation of regular expressions; however, given your example using HTML, I presume you may really be looking for HTML::Template and similar modules.