Use a 'substitute' ;-) 
$s1 = '0000010000000100000100001';
$s1 =~ s/1(0+)1/'1'x((length($1)+2))/eg;

[download]
Output 
 0000011111111100000111111

In reply to Re: Creating a fill-in regexp by shenme
in thread Creating a fill-in regexp by jpfarmer

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