# Pull out two dates from line 12 with a global match of
# 2 digits, a space, three letters, a space 4 digits. The
# round brackets allow you to capture what matches inside
# them.
#
my ($startPeriod, $endPeriod) =
   $items[10] =~ m{(\d\d\s[A-Za-z]{3}\s\d{4})}g;

# Transform date by capturing (round brackets) the day,
# month and last 2 digits of the year in $1, $2 and $3
# then concatenating them; the 'e' flag after the
# regular expression tells the regex engine to execute
# the code to compute the substituting string. The '.'
# is the string concatenation operator.
#
($fld11 = $startPeriod)
   =~ s{(\d\d)\s([A-Za-z]{3})\s\d\d(\d\d)}{$1 . $2 . $3}e;
($fld12 = $endPeriod)
   =~ s{(\d\d)\s([A-Za-z]{3})\s\d\d(\d\d)}{$1 . $2 . $3}e;