Can anybody help me understand what's going on here? Apparently, string interpolation is happening at some point where I did not expect it. I have a file with about 30 very long lines. The lines range from 50,000 to 70,000 chars each. I wrote a small bit of Perl expecting to cut out some short strings near the beginning of each line, but to keep the rest of each line, writing the result back to a new file. 
use strict;
my $me = "read52";

# Perl script to check (and revise?) the long lines in file save52.dat

my $file52 = "save52.dat";
my $newfile52 = "save52x.dat";

open( S52, "<$file52" ) or
  die "$me: Failure opening old long-lines file '$file52'.\n";

open( NS52, ">$newfile52" ) or
  die "$me: Failure opening new long-lines file '$newfile52'.\n";

while ( <S52> )
{
  chomp;
  my $new = substr( $_, 5, 9 ) . substr( $_, 53 );
  print NS52 "$new\n";
}
close S52;
close NS52;

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What happened surprised me. Apparently, interpolation occurred somewhere in the process of reading or writing the long lines. Here are two cases which I studied in detail. The substring 
((100% - 278px)/2)

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in the original became 
((100789f78<- 270 spaces ->x)/2)

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in the copy. where 
<- 270 spaces ->

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is a way of writing 
" " x 270

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The substring 
((100% - 206px)/2)

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in the original became 
((100789f78<- 198 spaces ->x)/2)

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in the copy. where 
<- 198 spaces ->

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is a way of writing 
" " x 198

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What happened?? Clearly, the percent character is suspicious. But when did it get its special meaning?

In reply to string interpolation by LloydRice

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