Because it is simple and efficient:

$s = 'variable chars anchor want this';;
$s = substr $s, index $s,'anchor';;
print $s;;
anchor want this
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If I was going to use a regex (say the anchor had a variable component), then:

$s = 'variable chars anchor want this';;
$s =~ s[.+(?=anchor)][];;
print $s;;
anchor want this
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The difference:

cmpthese -1,{
    a=> q[$s='variable chars anchor want this';$s=~s[.+(?=anchor)][];]
+,
    b=> q[$s='variable chars anchor want this';$s=substr $s, index $s,
+'anchor';]
};;
       Rate    a    b
a 1592446/s   -- -48%
b 3055291/s  92%   --
[download]