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if ( $variable !~ /^<exm>/ ) { $exmno = 0 );

if ( !/^<exm>/ ) { $exmno = 0 };

/^<exm>/ or $exmno = 0;

my $exmno = 0;
while(<>) {
    s/^<exm>/'<exm num ='.++$exmno.'>'/ or $exmno = 0;
    print $_;
}