Perl keeps the position of regexp-matching in mind: bound to a variable! See:

my $var = 'a111a222a333';
$var =~ m/a(\d+)/g and print $1
# prints '111'
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The regex ended after the three 1es, at position 4. This position ist stored by perl inside tha variable. You can retrieve it via pos($var). Read on it in `perldoc -f pos`

$var =~ m/a(\d+)/g and print $1;
#will now print '222' because the regex starts at
# pos($var) (which is 4)

#You can also use pos() as an lvalue
pos($var) = 0;
$var =~ m/a(\d+)/ and print $1;
# NO, doesn't print '333'
# but '111' because pos($var) wat set to 0 and so
# the regexp startsat positin 0 again
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Why do I tell you so? If you generate the strng again, the pos() will allways go back to 0, because you do not always match on the same variable, but on a new variable each time.