Replacing the number starting with one or more zeros and followed by a number with none. Here (?=\d), is positive look ahead which has zero width assertion. So it ll replace only the zeroes and not the following number.

Take a look at perlre and YAPE::Regex::Explain

For example:

$str = '004asdfsa'; #string starting with zero and followed by numbers

$str =~ s/^0+(?=\d)//;

print $str;

Output:
--------
4asdfsa
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$str = 'a004asdfsa'; #not starting with zero

$str =~ s/^0+(?=\d)//;

print $str;

Output:
--------
a004asdfsa
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$str = '0a4sdfsa'; #no number followed by zero

$str =~ s/^0+(?=\d)// ;

print $str;

Output:
--------
0a4sdfsa
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