Without /g it would be an infinite loop. But with /g, the matching starts where the last one stopped: 
$_ = "abcabcabc";
while (/c/g && /a/g) {
    say pos;
}

__END__
Output:
4
7

[download]
($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

[download]

In reply to Re^3: Open several files and read line by line by choroba
in thread Open several files and read line by line by Leiria

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