Here are two options. As a script:

use strict;
use warnings;

while (<>) {
    print "$1\n" if /(>.+?)\|/;
}

Usage: perl script.pl inFile [>outFile]

The last, optional parameter directs output to a file.

As a one liner:

perl -lne 'print $1 if /(>.+?)\|/' inFile [>outFile]

Output from both on your dataset:

>ENSMUSG0000005892
>ENSMUSG0000001537
>ENSMUSG00000002234237

In both cases, the regex captures all the characters starting with ">" up to the first "|", and then the results are printed.

Hope this helps!

while(<stdin>){
if ($_ =~/>/){
@arr=split ($_, "I")
print $arr[0]
}
}

I left on you to make this code run-able!! :):)

this will work on unix also

grep ">" file.txt | cut -d "|" -f 1

But always try to learn Friend !! :)