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a/list.txt a/list.txt 
a/list.txt b/list.txt 
...
a/list.txt i/list.txt 
a/list.txt j/list.txt

######## CODE.PL #############
#!/usr/bin/perl
...
    }
    return %new_hash;
}

##########INPUT###############
list.txt
...
product[s]: 3182
substrate[s]: 3186
product[s]: 2809

############OUTPUT#################
Processing file: list.txt
...
    ^
    |
substrate