#! perl -slw
use strict;

while( <DATA> ) {
    chomp;
    print;
    if( /the line/ ) {
        my $backup = -length() + 2;
        print "Backing up $backup places";
        seek DATA, $backup, 1;
        chomp( $_ = <DATA> );
        print "?$_?";
    }
}

=output
c:\test>junk
junk
junk
more junk
the line
Backing up -6 places
?the line?
other junk
yet more junk
=cut
__DATA__
junk
junk
more junk
the line
other junk
yet more junk