Your problem lies with understanding the debugger. If you do a step it executes the previous shown line and shows you the next. For example

> cat t7.pl
#!/usr/bin/perl -w
$t=1;
$t=5;
$t=10;

> perl -d t7.pl

main::(t7.pl:7):        my $t=1;
  DB<1> x $t
0  undef
  DB<2> s
main::(t7.pl:8):        $t=5;
  DB<2> x $t
0  1
  DB<3> q
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So the line ($p, $data) = @_; is the one that changes your var