prasadbabu uses $pre= $`;$post= $'; that is bad for the performance for your script, because it slows down the regex-engine.
$gubes code can be optimized, no need for unpack():

$str='This is Sample Pre Match Text<web>www.hotmail.com</web>This is S
+ample Post Match Text';
($pre,$post)=$str =~ m#(.{1,15})<web>.*</web>(.{1,15})#gsi;
print "\n$pre";
print "\n$post"
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